I was having the same issues. Delete just the gmail app from your application list, let the blackberry restart and then go to m.google.com and download just the gmail app. Everything should work just fine then.
I've installed that on a download before and it worked really well. If Chrisy's Cache Clearing (say that 10 times fast!) doesn't work, you might installing through DM.
Posted from my CrackBerry at wapforums.crackberry.com
Glad I could help, I had posted a thread yesterday about the same problem and someone told me to do that. I tried to copy my thread so that you could read it, but I don't have 10 posts to put links in my threads.